In recent years, with the spread of portable electronic equipment such as portable telephones and notebook-sized personal computers, there is a strong need for development of a compact and lightweight nonaqueous-electrolyte secondary battery having high energy density. There is also a strong need for development of a high-output secondary battery as a power source for driving a motor, and particularly as a battery of the power source of transport equipment.
As a secondary battery that satisfies such a demand, there is a lithium ion secondary battery which is a nonaqueous-electrolyte secondary battery. A nonaqueous-electrolyte secondary battery includes an anode, a cathode, an electrolyte and the like, and as an active material for the anode and cathode, a material capable of insertion and desorption of lithium ion is used.
Currently, as the cathode material of this kind of nonaqueous-electrolyte secondary battery, lithium transition metal containing composite oxide such as lithium cobalt composite oxide (LiCoO2) for which synthesis is relatively easy, lithium nickel composite oxide (LiNiO2) that uses nickel that is less expensive than cobalt, lithium nickel cobalt manganese composite oxide (LiNi1/3Co1/3Mn1/3O2), lithium manganese composite oxide (LiMn2O4) that use manganese, and lithium nickel manganese composite oxide (LiNi0.5Mn0.5O2) have been proposed.
It is required for a nonaqueous-electrolyte secondary battery to have characteristics such as high capacity, output characteristics, retention characteristics, and cycle characteristics. The characteristics of this kind of nonaqueous-electrolyte secondary battery are affected by the characteristics of the cathode active material used for its cathode material. In order to provide a nonaqueous-electrolyte secondary battery having this kind of excellent characteristics, many kinds of cathode active material have been proposed.
For example, JP2001-266876 (A) discloses a cathode active material comprising a lithium nickel containing composite oxide that is expressed by a general formula: LixNiyCoz Al(1−y−z)O2, where 0.05≤x≤1.10, 0.7≤y≤0.9, 0.05≤z≤0.18, and 0.85≤y+z≤0.98, a specific surface area thereof is 0.7 m2/g or less, and a tap density thereof is 2.3 g/ml or more. As this cathode active material has a stable crystal structure, by using this as a cathode active material, it is possible for a nonaqueous-electrolyte secondary battery to have a high capacity while improving the storage characteristics (performance retention characteristics in a storage state) under high-temperature conditions.
Further, JP2005-251716 (A) discloses a cathode active material comprising a lithium transition metal composite hydroxide having an excellent packing efficiency in order to improve characteristics of a nonaqueous-electrolyte secondary battery such as the thermal stability, load characteristics, output characteristics, by focusing on the structure of primary particles and preventing the generation of fine particles due to pulverization of the primary particles. This lithium transition metal composite oxide is being composed of particles of either or both primary particles and/or secondary particles that are aggregates of the primary particles, and has an aspect ratio of 1 to 1.8, and at least has an element selected from a group consisting of molybdenum, vanadium, tungsten, boron, and fluorine at least on its surface. Among these added elements, especially boron works as flux and promotes the crystal growth of the particles, and improves storage characteristics of a nonaqueous-electrolyte secondary battery using this cathode active material as a cathode material.
However, when boron is used as flux, as the amount of boron increases to even promote the crystal growth, boron remains as impurities and may cause problems such as deterioration of electrochemical characteristics.